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Interlaminar Shear Strength and Failure Analysis of Aluminium-Carbon Laminates with a Glass Fiber Interlayer After Moisture Absorption

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Interlaminar Shear Strength and Failure Analysis of Aluminium-Carbon Laminates with a Glass Fiber Interlayer After Moisture Absorption materials Article Interlaminar Shear Strength and Failure Analysis of Aluminium-Carbon Laminates with a Glass Fiber Interlayer after Moisture Absorption Jarosław Bienia´s*, Patryk Jakubczak , Magda Dro´zdzieland Barbara Surowska Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology, Nadbystrzycka 36, 20-618 Lublin, Poland; [email protected] (P.J.); [email protected] (M.D.); [email protected] (B.S.) * Correspondence: [email protected] Received: 11 May 2020; Accepted: 3 July 2020; Published: 6 July 2020 Abstract: This article presents selected aspects of an interlaminar shear strength and failure analysis of hybrid fiber metal laminates (FMLs) consisting of alternating layers of a 2024-T3 aluminium alloy and carbon fiber reinforced polymer. Particular attention is paid to the properties of the hybrid FMLs with an additional interlayer of glass composite at the metal-composite interface. The influence of hygrothermal conditioning, the interlaminar shear strength (short beam shear test), and the failure mode were investigated and discussed. It was found that fiber metal laminates can be classified as a material with significantly less adsorption than in the case of conventional composites. Introducing an additional layer of glass composite at the metal-composite interface and hygrothermal conditioning influence the decrease in the interlaminar shear strength. The major forms of damage to the laminates are delaminations in the layer of the carbon composite, at the metal-composite interface, and delaminations between the layers of glass and carbon composites. Keywords: fiber metal laminates; moisture absorption; interlaminar shear strength; failure modes 1. Introduction Among the fiber metal laminates (FMLs), the most noteworthy are the laminates where the layers are made of carbon reinforced aluminium laminates (CARALL). CARALL are characterized by impact resistance, a high fatigue strength, and a low density with very low growth rates and efficient crack bridging [1–7]. An important aspect of FMLs is their high resistance to corrosion. This phenomenon is caused by the composite layers acting as a barrier in the corrosion process, which is limited to the outer metal layers [1,8–10]. Nevertheless, the CARALL may be more prone to corrosion. In this case, corrosion resistance is especially influenced by the electrical conductivity of carbon fibers. As a result, galvanic interactions occur at the metal-composite interface, especially in wet environments [11–13]. The absorption of moisture may lead to a decrease in the glass transition temperature; a plasticizing matrix; swelling, cracking, and delaminations at the fiber-matrix interface, modification of the local stress state; and rupture of the adhesive bonding in the system [7,14–16]. It was previously shown that moisture absorption in polymer composites decreases the mechanical properties and interlaminar shear strength [17–20]. In the literature, articles on the influence of environmental conditions on the properties of fiber-metal laminates can be viewed. Botelho et al. [7] investigated the influence of hydrothermal conditioning on the mechanical properties of CARALL by conducting tensile and compression tests. The authors suggest that the changes in the mechanical properties of CARALL are negligible, similar to the case of glass reinforced aluminium (GLARE) laminates [17]. In another Materials 2020, 13, 2999; doi:10.3390/ma13132999 www.mdpi.com/journal/materials Materials 2020, 13, 2999 2 of 14 paper [21], the authors conducted research on hydrothermal effects by using the losipescu and interlaminar shear strength (ILSS) test for GALRE laminates. A decrease in shear strength was observed. However, it was stated that the influence of the hydrothermal effects on the shear strength results was inconsequential. The results of ILSS tests with a completely saturated GLARE specimen presented by Borgonje et al. [9] show that the presence of moisture has a significant influence on the obtained strength values. After exposure to a high-humidity environment, they observed a significant decrease in the tensile strength. In other studies, it was found that hygrothermal conditioning reduces the shear strength of CARALL laminates due to moisture absorption [5]. A decrease of the mechanical properties in CARALL after hygrothermal ageing was also observed by Pan et al. [10]. Research [22] on the effect of hydrothermal conditioning on the mechanical properties of fiber metal laminates such as GLARE and CARALL indicates a decrease in the tension resistance and ILSS of 15% and 9–11%, respectively, depending on the configuration. Ali et al. [15] investigated the seawater hydrothermal conditioning effects on the properties of titanium-carbon fiber/epoxy FMLs for marine applications. Their results showed that the percentage reduction in the mechanical properties of the Ti FML was lower in comparison to the conventional composites. The good properties of titanium fiber metal laminates after environmental conditioning in comparison to other FMLs were also confirmed by Silva et al. [23]. Poodts et al. [24] characterized a fiber metal laminate (with steel layers) for underwater applications. They observed that steel layers offer very good protection against water absorption and prevent the loss of the mechanical properties associated with it. Their results showed that saturated specimens exhibit a loss of the flexural modulus and shear strength of approximately 20%. It is very important to maintain proper bonding between the matrix and the steel layer. The authors found that water penetration at the steel-matrix interface induces delaminations, with subsequent degradation of the component. Accurate manufacturing processes ensuring a perfect bond between the steel layer and the matrix are thus strongly recommended. Singh and Angra [25] also studied the hygrothermal degradation of stainless steel fiber metal laminate conventional GF/E (glass fibre/epoxy) composites. In water, the compressive and tensile strengths of FMLs were reduced by 32.6% and 23.4%, respectively. The highest reduction in the compressive and tensile strengths of the GF/E composite was recorded as 36.8% and 29.8%, respectively, in water. Hygrothermally and unconditioned and conditioned tensile specimens fail due to delamination and fiber breakage, respectively, while compression specimens fail because of delamination between GF/E layers. Park et al. [26] investigated the effects of void contents on the long-term hydrothermal behaviors of GLARE laminates. They observed that the voids contribute to the water absorption properties and that the decrease in ILSS depends on the characteristic of water adsorption and voids. Large ILSS degradations of between 17% and 32% occurred for GLARE in comparison to the non-aged counterparts. Chandrasekar et al. [27] presented an experimental review on the mechanical properties and hydrothermal behavior of fiber metal laminates. Botelho et al. [28] also researched the hydrothermal effects on viscoelastic properties, such as the storage modulus (E0) and loss modulus (E”), for glass fiber/epoxy/aluminum laminate (GLARE). For GLARE laminates, the E0 modulus remains unchanged (49 GPa) during the cycle of hydrothermal conditioning. The research conducted by Zong et al. [29] showed a significant decrease in both the fatigue life and the tensile strength of Glare 4A laminates after being hygrothermally aged. Hu et al. [30] studied the long-term moisture absorption behavior of carbon-fiber reinforced polyimide (CF/P) and polyimide–titanium-based fiber metal laminates. The fiber metal laminates exhibited a slower moisture absorption rate in comparison to the pure composite. This was a result of the barrier function of the metal layers at both surfaces of the fiber metal laminates. The interlaminar and flexural strength of CF/P and fiber metal laminates decreased after hygrothermal ageing exposure. The mechanical properties of CF/P composites were reduced more than those of the fiber metal laminates. In the fiber metal laminates, debonding was only observed in the edge area. Molerio et al. [31] developed a new layer-wise mixed model for a coupled hygro-thermo-mechanical static analysis of fiber metal laminates, under a series of hygro-thermo-mechanical loadings. In another work, Molerio et al. [32] presented three-dimensional exact hygro-thermo-elastic solutions for multilayered plates: composite laminates, Materials 2020, 13, 2999 3 of 14 fiber metal laminates, and sandwich plates. These effects are demonstrated by the through-thickness distributions of displacements, stresses, temperatures, and weight percent moisture contents for the selected multilayered plates under different hygro-thermo-mechanical loadings, which may serve as 3D benchmark exact solutions. Works focused on improving the interlaminar strength can also be noted. For example, Garcia and co-authors [33] studied the effect of nylon nanofibers on the delamination resistance and dynamic behavior of GFRP composites. Due to the incorporation of nylon nanofibers in the nano-modified composites, a significant (27%) increase in the inter-laminar shear strength was obtained. This can be related to the presence of significant bridging phenomena due to the interaction between the nylon nanofibers and the epoxy resin. The authors concluded that improvement of the delamination resistance of GFRP laminates can be obtained by using nylon nano-fibers. In general, despite a significant resistance to environmental factors, moisture absorption may significantly
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